Process for preparing filterable polystyrene dispersion

ABSTRACT

Particulate, filterable polystyrene formulations are obtained by anionic polymerization of styrene in nonaqueous dispersion if the polymerization is carried out in an aliphatic hydrocarbon as the dispersing medium and with a styrene/diene/styrene three-block copolymer, for example based on polybutadiene or polyisoprene, as the dispersant.

Styrene polymers are used in particular for consumer products. They canbe used alone or in the form of polymer blends with other thermoplasticpolymers for the production of materials having many advantageousproperties. Polystyrene is chemically inert to a large number ofchemical compounds and has excellent proccessability. In general, thephysical properties and the external form of styrene polymers depend onthe method of their preparation.

The present invention relates to a process for the preparation ofparticulate polystyrene (for example in the form of a latex or powder)by anionic polymerization of styrene which is dispersed in a hydrocarbonnonsolvent, a special styrene/butadiene block copolymer being used asthe dispersant.

There is considerable interest in particulate polymers. In some cases,attempts are made to obtain polymers having a particularly narrowmolecular weight distribution and a particle size of more than 200 μm.As described in U.S. Pat. No. 4,247,434, polymer microparticles in latexform are used in many applications, for example as a reference standardfor the calibration of various instruments in medical research.Polystyrene in particle form is once again of particular interest here.

U.S. Pat. No. 3,645,959 describes the preparation of high molecularweight vinyl polymers, for example polystyrene, in nonaqueousdispersion. Up to one percent of a diolefin, for example norbornadiene,is added as a dispersant for this purpose.

U.S. Pat. No. 3,259,595 describes a polystyrene preparation startingfrom a suspension polymerization, which polystyrene preparation can beprocessed to a polymer film having an approximately cellular structure.

U.S. Pat. No. 4,029,869 describes a method for the preparation ofpolystyrene by free radical suspension polymerization (initiated byperoxide). Very broad molecular weight distributions are obtained.

The anionic emulsion polymerization permits the preparation ofpolstyrene dispersions having a narrow molecular weight distribution.This method is described, for example, in European Patent 290,195 andU.S. Pat. Nos. 4,942,209 and 4,871,814; styrene/diene two-blockcopolymers are used as dispersants.

Polystyrene particles which were prepared by means of anionic emulsionpolymerization with the aid of such a two-block copolymer had a particlesize of less than 15 μm and proved to be non-filterable since suchparticles are not retained even by very fine filter cloths and blockedthe finest filters used by way of experiment. Where it was possible toobtain such fine polystyrene particles from the dispersion on the pilotscale, the powders obtained could be very readily swirled up, so thatproduction on the industrial scale would have given rise to the dangerof dust explosions.

We have found that, in the anionic emulsion polymerization of styrene,polystyrene particles which have a diameter of more than 100 μm,preferably from 200 to 1000 μm, and which have proven capable of beingfiltered are formed as a result of the use of astyrene/butadiene/styrene (three-)block polymer as the dispersant. Suchpolystyrene dispersions avoid the danger of a dust explosion since thefilter cake obtained can, if necessary, be processed directly to give anextrudate.

The present invention relates directly to a process for the preparationof polystyrene by anionic polymerization of styrene in nonaqueousdispersion, which is carried out in an aliphatic hydrocarbon as thedispersing medium and with a styrene/diene/styrene three-block copolymeras the dispersant.

The styrene/diene/styrene three-block copolymer to be used according tothe invention is used, for example, in an amount of from 0.1 to 10% byweight, based on styrene, and the diene block may be hydrogenated orunhydrogenated. Suitable copolymers are commercially available and canbe used directly for carrying out the invention.

Instead of copolymers having polybutadiene blocks, it is also possibleto use those which contain polymerized polyisoprene, and these in turnmay be hydrogenated or unhydrogenated.

The dispersing medium used is, for example, an alkane of 4 to 8 carbonatoms, preferably a butane, pentane or hexane, or a hydrocarbon mixtureof, for example, 4 to 8 carbon atoms.

The polymerization temperature is in general from 50° to 100° C. and thedispersion used contains, for example, from 20 to 60% of styrene.

The amount of initiator is chosen in a known manner so that thepolystyrene to be prepared has a molecular weight of from 5000 to10,000,000 g/mol; the method is therefore not restricted with regard tothe obtainable molecular weights. It is found that the polystyreneobtained generally has a molecular nonuniformity (distribution width ofthe molecular weight) M_(w) /M_(n) of less than 2.

In the novel process, it is possible to obtain filterable polystyrenedispersions by an anionic method. While the process known to date, inwhich styrene/butadiene two-block polymers are used as dispersants,gives only particles which have a size of less than 5 μm and blockscreens or filter materials, the invention gives substantially largerparticles which furthermore have a desirably narrow molecular weightdistribution (M_(w) /M_(n) preferably less than 2). The noveldispersions can readily be converted by filtration into correspondingconcentrates (latices). and, at higher concentration and if thepreparation of, for example, extrudates is desired, corresponding filtercakes, which can also be dried; after the dispersing medium has beenseparated off, there is no longer any danger of a dust explosion.

The polymerization can be carried out, for example, as follows:

A styrene/butadiene/styrene (three-)block polymer which has a styreneblock, an unhydrogenated or a hydrogenated butadiene middle block andanother styrene block is dissolved in styrene to give an about 0.5-20%strength by weight solution, based on styrene. A commercial blockpolymer having a suitable structure, for example one of the brandsavailable under the trade name Kraton® G (Shell Chemie), may be used.This solution is mixed with, for example, pentane and heated to, forexample, 30° C. All impurities are pretitrated with a dilute solution ofa conventional initiator (eg. sec-butyllithium) until a permanent yellowcolor is obtained. The calculated amount of initiator is then added andthe mixture is heated to an internal temperature of from 65° to 70° C.The polymerization is allowed to go to completion (about 1 hour) and isstopped with methanol.

The dispersion can be freed from the dispersing medium by filtration,and the particles obtained can be dried under reduced pressure.

Regarding the other details of the anionic emulsion polymerizationmethod, reference may be made to the abovementioned publications in thisarea.

The novel products can be mixed with polymers as described in KunststoffKompendium by Franck and Biederbick, Vogel-Verlag, 1990. The novelproducts are preferably mixed with styrene poilymers and styrenecopolymers. Examples of these are S/B copolymers and styrene/butadieneblock copolymers (two-, three-or four-block copolymers or multiblockcopolymers), it also being possible for the diene component to behydrogenated. Instead of butadiene, it is also possible to use isopreneor another 1,3-diene. In the styrene homopolymers, general purposepolystyrene is preferred. Surprisingly, an improvement in the mechanicalproperties is found in the case of blends with a general purposepolystyrene and the novel products. The novel high molecular weightpolystyrene is readily compatible with the general purpose polymers and,for example, improves the yield stress, the modulus of elasticity andthe toughness.

The blends are prepared in solution or mechanically (for example bymeans of an extruder).

COMPARATIVE EXPERIMENT

A mixture of 50% by weight of polystyrene 165 H from BASF AG and 50% byweight of polystyrene 486 M was dissolved in toluene and precipitatedwith methanol and dried at 110° C. Thereafter, test specimens wereinjection molded at a melt temperature of 220° C. and the mechanicalprofile of the blend was tested. The tensile test according to ISO 527gave the following values: tensile modulus of elasticity: 2346 MPa;tensile strength: 32 MPa; nominal elongation at break: 30.9%; Charpyimpact strength according to ISO 179/1Eu 157.4.

EXAMPLE 2

A mixture of 40% by weight of polystyrene 165 H from BASF AG and 50% byweight of polystyrene 486 M and 10% by weight of the novel polystyrenehaving a molecular weight M_(w) of 1,000,000 g/mol (D=1.1) was dissolvedin toluene and precipitated with methanol and dried at 110° C.Thereafter, test specimens were injection molded at a melt temperatureof 220° C. and the mechanical profile of the blend was tested. Thetensile test according to ISO 527 gave the following values: tensilemodulus of elasticity: 2841 MPa; tensile strength: 47 MPa; nominalelongation at break: 38.2%; Charpy impact strength according to ISO179/1Eu 204.0.

EXAMPLE 3

4.55 g of an S-B-S copolymer commercially available under the nameKraton® G 1652 from Shell Chemicals were dissolved in 182 g of styreneunder nitrogen and the solution was stirred into 330 g of n-hexane. Allimpurities were pretitrated with sec-butyllithium, and thepolymerization was initiated at 26° C. by adding 0.73 mmol ofsec-butyllithium. The temperature was regulated to 63° C. and themixture was left at this temperature for 90 minutes. To stop thereaction, an excess (10 ml) of isopropanol was added. The dispersionobtained was filtered by means of a glass filter D4 (pore size 10-16 μm,manufacturer Schott, Mainz) and was dried in a drying oven at 65° C.under reduced pressure for 12 hours. The conversion was more than 99%. Asample was suspended in immersion oil and was examined between two coverplates in transmitted light under the microscope (Zeiss Axiophot); theparticles had a size from about 350 to 700 μm. M_(w), determined by gelpermeation chromatography, was 250,000 g/mol and M_(w) /M_(n) was 1.19.

COMPARATIVE EXPERIMENT

4.55 g of an S-B two-block copolymer commercially available under thename Nippon® NS 312S from Nippon Zeon were dissolved in 182 g of styreneunder nitrogen and the solution was stirred into 330 g of n-hexane.After pretitration of the impurities with se,c-butyllithium, thepolymerization was initiated at 26° C. by adding 1.82 mmol ofsec-butyllithium and a further procedure was carried out as above. Thedispersion proved to be nonfilterable; it was allowed to settle out andwas decanted, and the residue was dried in a drying oven at 65° C. underreduced pressure for 12 hours. The conversion was more than 99%; theparticle size was about 6 μm, M_(w) was 140,000 g/mol and M_(w) /M.sub.was 1.16.

We claim:
 1. A process for the preparation of polystyrene by anionicpolymerization of styrene in nonaqueous dispersion, wherein thepolymerization is carried out in an aliphatic hydrocarbon as thedispersing medium and with a styrene/diene/styrene three-block copolymeras the dispersant.
 2. A process as claimed in claim 1, wherein thestyrene/diene/styrene three-block copolymer is used in an amount of from0.1 to 10% by weight, based on styrene.
 3. A process as claimed in claim1, wherein the block copolymer contains polymerized polybutadiene orpolyisoprene and the diene polymer is hydrogenated or unhydrogenated. 4.A process as claimed in claim 1, wherein the dispersing medium used isan alkane of 4 to 8 carbon atoms.
 5. A process as claimed in claim 4,wherein a butane, pentane or hexane or a mixture of alkanes of 4 to 8carbon atoms is used.
 6. A process as claimed in claim 1, wherein thepolymerization temperature is from 50° to 100° C.
 7. A process asclaimed in claim 1, wherein a dispersion which contains from 20 to 60%by weight of styrene is used.
 8. A process as claimed in claim 1,wherein the amount of initiator used corresponds to a molecular weightof from 5000 to 10,000,000 g/mol for the polystyrene to be prepared. 9.A process as claimed in claim 1, wherein the procedure is chosen so thatthe polystyrene obtained has a molecular nonuniformity (distributionwidth of the molecular weight) M_(w) /M_(n) of less than 2.